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Sculpting quantum reality

You may think you know what is real. In fact most of the time the question seems to be clear-cut: the chair you're sitting on surely exists, as does the screen where you read these words. Or at least that seems to be a sensible way to think about things. But when talking about the world of the very small, like atoms or fundamental particles, the question is not only less clear, it is vexing.

In fact, at the root of one of our best theories theories of Nature, quantum physics, there is a reality crisis. Many interpretations exist which are equally compatible with the theory, but paint very different pictures of reality. Some of them say that there are parallel universes, others say that everything in the world is connected instantaneously, and yet others say that quantum theory is fundamentally about conscious experience.

One of the great conundrums of quantum theory is that it is so successful and yet we don't really know what it means. - Dr Eric Cavalcanti

These debates about quantum reality are often purely theoretical, but researchers at the University of Queensland and University of Sydney, have recently gathered results, to be published on Nature Physics, that rules out a class of such interpretations.

At the root of quantum physics is an object called the quantum state. Within some interpretations, the quantum state is a real object, something out there in the world. Thinking of the quantum state as a real object leads to bizarre consequences, however, such as Schrödinger's famous cat, which can be dead and alive at the same time, in a state that is called a superposition. This has led some physicists, including Einstein, to suggest that the quantum state doesn't give us a picture of the way the world is, but is only a mathematical representation of our knowledge about the world.

But the experiment lends strong evidence against Einstein's view. Cavalcanti offers an analogy: "When asked about how he sculpted David, Michelangelo reportedly replied 'It's simple - I just remove the parts which are not David.' What we are trying to do is to remove the parts which are not reality. Ultimately, it may well be that we won't be able to tell between some of the interpretations -- the David of quantum theory might be fundamentally incomplete. But we can rule some of them out, and reduce the mystery at least by that little bit."

"To perform that test, we prepare light particles and subject them to very precise measurements," Dr Martin Ringbauer from the University of Queensland explains. "A central feature of quantum mechanics is that we cannot tell those states apart with certainty. Our experiment shows that the knowledge interpretation of the wave function cannot fully explain this."

In other words, the real state of Schrödinger's cat would indeed be a superposition of being dead and alive. Which could mean there are in fact infinitely many quantum universes.

Or maybe there was never a cat in the first place.

Apart from the fundamental implications, this question of whether quantum states are grounded in reality or not is an important one for anyone trying to use the 'weirdness' of the quantum world for new technologies or form the basis of a quantum computer.

"Our result can be understood as arising from a constraint on certain classical simulations of quantum systems. By showing that quantum physics can in fact go beyond those limitations, we demonstrate a new way in which quantum resources can outperform classical ones", says Eric Cavalcanti.

The Nature paper can be found here and the New Scientist article on the paper can be found on the New Scientist website